Fine biography prunes the thicket of myths surrounding Ty Cobb; Ty Cobb, by Charles C. Alexander. New York: Oxford University Press. 272 pp. $ 16.95

THE last lines of Charles C. Alexander's quite good biography of Ty Cobb tell us ''he was never an easy man to know, never easy to get along with in or out of uniform, never really at peace with himself or the world around him.''

''Ty Cobb,'' says Alexander, ''was the most volatile, the most fear-inspiring presence ever to appear on a baseball field. His equal is not likely to come along again.''

Alexander is probably right, for even Pete Rose pales in comparison to the extraordinarily intense Cobb, a player who changed the way the game was played so obviously that most baseball historians think of the early years of our century as ''Cobbian.''

Tyrus Raymond Cobb was born Dec. 18, 1886, in Georgia; his father, a schoolmaster and Georgia state senator, hoped his son would enter law or medicine. When Cobb insisted on baseball, his father insisted only that he not ''come home a failure.''

Cobb made the major leagues, with the Detroit Tigers, in 1905, beginning a 24 -year career that would earn him virtually no friends and countless records, a few of which - especially his career 4,191 hits - still stand.

But the Ty Cobb of the small and inevitably hagiographic biographies I read in grade school is not the same Ty Cobb who appears in the pages of Alexander's book.

Cobb was a racist; he would fight with almost anyone; and he frequently carried a gun. In 1912, driving with his wife to the train station in Detroit, Cobb was accosted by three would-be robbers. Here, according to Alexander, is what happened:

''Cobb slid over his wife . . . and started exchanging punches with all three assailants. One of them pulled a knife and slashed his back . . . Cobb . . . caught one mugger, clubbed him with a big Belgian revolver he usually carried, and then trapped another at the end of an alley. Cobb beat the man senseless with the revolver and left him there. When he boarded the train, a teammate noticed the blood-soaked back of his coat. Trainer Tuthill cleaned and bandaged the knife wound. Cobb got three hits at Syracuse the next day.''

Cobb was also the first millionaire baseball player. Alexander tells us that ''scanning the stock listings in his morning newspaper had long been part of his daily routine, and wherever he happened to be, he kept in frequent contact with his brokers.''

It also seems that no one liked Cobb. Teammates respected his abilities, but did not want to play with him; other teams frequently complained about his aggressive tactics, especially on the base paths, where he went in high, spikes flashing. His marriages all folded; he was unsuccessful as a manager.

To Alexander's credit, he does not present all the evidence in ''Ty Cobb'' as an indictment, but more as fact. It is clear that Alexander is a fan, and his admiration of Cobb the ballplayer large. But he is also a historian interested in pruning the thicket of myth surrounding Cobb. The result is fine biography.